People are attracted to cities because of the job opportunities that they bring. At the same time, house prices tend to push people away from urban centres. The draw of healthy living and green environments similarly pulls people towards less urban environments, further from city centres. The limiting factor in suburban spread – the expansion of cities beyond their existing boundaries – is often travel time, by either public or private means. One of the appeals of driverless cars is that they are set to cut delays by up to 40 per cent because they switch lanes more efficiently, drive closer to the vehicle in front, and travel at a constant speed without repeatedly braking and accelerating, which is the main cause of congestion.